In recent years, nonaqueous electrolyte secondary batteries having high energy density and excellent in cycle performance have been drawing attention as power sources for portable appliances such as mobile phones and notebook personal computers as well as for electric automobiles. Among such nonaqueous electrolyte secondary batteries, those which are presently most widely distributed in the market are compact batteries for consumer use represented by batteries of 2 Ah or less for mobile phones.
Today, there are a lot of materials as a positive electrode material for a nonaqueous electrolyte secondary battery and most commonly known are lithium-containing transition metal oxides mainly containing lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) having an operating voltage around 4V as well as lithium manganese oxide (LiMn2O4) having a spinel structure. Especially, lithium cobalt oxide among them has been widely employed as a positive electrode material excellent in charge-discharge characteristics and energy density in a small capacity lithium secondary battery with a battery capacity up to 2 Ah.
However, in consideration of development of middle to large scale batteries for industrial use expected to be strongly demand in future, safety of batteries will be regarded as very important and the specifications of today's compact batteries can not satisfy the required safety. One of the reasons for this problem is that the thermal stability of the positive electrode material of the compact batteries is low.
Therefore, recently, a positive electrode material containing a polyanion with high thermal stability such as LiFePO4 having an olivine structure has been drawing attention among lithium-containing composite oxides. This material does not release oxygen even at a high temperature and is thus suitable for remarkably improving the safety of batteries.
However, the positive electrode material containing a polyanion such as LiFePO4 has low electric conductivity as compared with a lithium-containing transition metal oxide such as LiCoO2 and therefore, if an electrode is produced in the same manner as in LiCoO2, the battery to be obtained can not give sufficient high rate discharge characteristics.
To improve such a defect, Patent Documents 1 to 10 disclose techniques of forming carbon coatings on particle surfaces of polyanion materials such as LiFePO4.
Japanese Patent Application Laid-Open (JP-A) No. 2001-015111 (Patent Document 1), a Japanese patent document, discloses a technique of obtaining lithium iron phosphate (LiFePO4) coated with a carbonaceous deposit by finely pulverizing run steel (Fe3(PO4)2·8H2O), lithium orthophosphate, and a polypropylene powder with a zirconia ball mill and thereafter heating the pulverized product at 350 to 700° C.
JP-A No. 2002-117833 (Patent Document 2), a Japanese patent document, discloses a technique of obtaining a LiFePO4/carbon composite body by mixing Li3PO4 and Fe3(PO4)2·8H2O, adding an acetylene black powder thereto to obtain a mixture, milling the mixture using a planet ball mill, and thereafter firing the mixture at 600° C.
JP-A No. 2003-034534 (Patent Document 3), a Japanese patent document, discloses a method for producing a carbon-containing lithium-iron composite oxide for a positive electrode material for a lithium secondary battery by mixing a lithium compound, an iron compound, a phosphorus-containing ammonium salt, and carbonaceous fine particles to obtain a mixture and firing the mixture at a temperature of 600 to 750° C., thereby making a composite of the carbonaceous fine particles with particles of the olivine structure lithium-iron composite oxide (LiFePO4).
JP-A No. 2003-292308 (Patent Document 4), a Japanese patent document, discloses a method for producing a lithium-iron-phosphorus type composite oxide-carbon composite body obtained by coating the surface of LiFePO4 particles with a conductive carbon material by mixing ferrous phosphate hydrated salt (Fe3(PO4)2·8H2O), lithium phosphate (Li3PO4), and a conductive carbon material, dry-pulverizing the mixture to obtain a reaction precursor with a specific volume of 1.5 ml/g or less, firing the reaction precursor to coat the surfaces of the LiFePO4 particles with the conductive carbon material, and pulverizing the particles.
JP-A No. 2004-186075 (Patent Document 5), a Japanese patent document, discloses a technique of coating the surface of a lithium iron oxide, which is a positive electrode material for a nonaqueous secondary battery, with carbon fibers.
Japanese Patent Application National Publication No. 2004-509058 (Patent Document 6), a Japanese patent document, and Japanese Patent Application National Publication No. 2004-509447 (Patent Document 7), a Japanese patent document, disclose methods for synthesizing positive electrode materials having cores of LiMPO4 coated with carbon by mixing an element M source, a lithium compound, and a compound to be a P source and thermally decomposing a carbon source in the presence of a conductive carbon source containing a furfuryl alcohol polymer.
US20040157126A1 (Patent Document 8) discloses a method for synthesizing a positive electrode material having cores of LiMPO4 coated with carbon by thermal decomposition of a gas mixture containing hydrocarbon as a carbon source.
Further, JP-A No. 2005-116393 (Patent Document 9), a Japanese patent document, discloses a method for making ultrafine particles of lithium iron phosphate as a means for improving the high rate discharge characteristics and discloses that it is easy to produce nano-particles with high purity and even composition as well as an average particle size of several 10 nm or smaller.
Furthermore, JP-A No. 2004-014340 (Patent Document 10), a Japanese patent document, discloses that the electron supply capability is improved by adding a substance having electric conductivity to the inside of a material such as a LiFe1−xMxPO4-based material. This document further discloses that a plurality of primary particles of the electrode material are aggregated and these primary particles are bonded to a three-dimensional mesh-like electric conductive substance layer in a thin layer state and made to form secondary particles with a spherical or polygonal shape as a whole. Moreover, carbon is disclosed as a substance having the electric conductivity.    Patent Document 1: JP-A No. 2001-015111    Patent Document 2: JP-A No. 2002-117833    Patent Document 3: JP-A No. 2003-034534    Patent Document 4: JP-A No. 2003-292308    Patent Document 5: JP-A No. 2004-186075    Patent Document 6: Japanese Patent Application National Publication No. 2004-509058    Patent Document 7: Japanese Patent Application National Publication No. 2004-509447    Patent Document 8: US 20040157126A1    Patent Document 9: JP-A No. 2005-116393    Patent Document 10: JP-A No. 2004-014340